Ion implanters are commonly used in the manufacture of semiconductor and metallic products for implanting ions into semiconductor or metallic substrates to change the conductivity of the material in such substrates or in predefined regions thereof. Ion implanters generally comprise an ion beam generator for generating a beam of ions, a mass analyser for selecting a particular species of ions in the ion beam and means to direct the ion beam through a vacuum chamber onto a target substrate supported on a substrate holder. The ion beam cross-sectional area depends on such factors as the beam line configuration, the degree of focusing applied to the ion beam, the gas pressure along the beam line, the energy of the ion beam and the mass of the ions. Most frequently, the ion beam cross-sectional area at the target substrate is less than the surface area of the substrate which necessitates traversal of the beam over the substrate in a one or two-dimensional scan so that the beam covers the whole surface thereof. The three two-dimensional scanning techniques commonly employed in ion implantation are (i) electrostatic and/or magnetic deflection of the ion beam relative to a static substrate, (ii) mechanical scanning of the target substrate in two dimensions relative to a static ion beam and (iii) a hybrid technique involving magnetic or electrostatic deflection of the ion beam in one direction and mechanical scanning of the target substrate in another generally orthogonal direction.
An important objective in the fabrication of semiconductor wafers is to ensure that for any selected species of ions, the wafers are implanted with the correct ion dose and that the dose is uniform throughout and across the wafer or part of the wafer targeted to receive the implanted ions. At present, the semiconductor industry frequently demands a dose uniformity of about 1% or better. Failure to achieve such standards is both time consuming and very costly due to the significantly high cost of the wafers themselves.
Dose is monitored by measuring beam current during an implant process using an ion beam current detector (usually a Faraday cup) positioned `behind` the plane of the wafer so that, as the beam and the wafer effect movement one relative to the other, the beam can fall on the Faraday cup. Where implantation of multiple wafers is concerned, this may be achieved by positioning the cup behind the movable (usually rotatably) wafer holder with one or more gaps/slits in the holder through which the beam can pass to the cup which is aligned with the general path of the beam. Where single wafer implantation occurs, the Faraday cup will normally be placed to one side of the wafer so that the beam is moved across the cup during each single traverse or sweep of the beam across the wafer.
Existing beam current detectors will not detect any ions which have been neutralised prior to being implanted in the wafer, and so will normally understate the true rate of delivery of desired species, including both ions and neutrals, in the beam. Beam ions are generally neutralised by collisions with residual gas molecules in the chamber and it is known that the proportion of ions which become neutralised increases with increasing residual gas pressure. Collisions may also result in the state of charge of beam ions being increased, e.g. from singly to doubly charged or reduced, e.g. from doubly to singly charged, and both these effects can contribute to beam current measuring errors.
It has been recognised that there is a need to compensate for these effects in measuring and calculating the true beam current, i.e. the current assuming no neutrals or changes of charge state. This true or corrected beam current would be a proper measure of the rate of delivery in the beam of particles of the species to be implanted. With accurate monitoring of corrected beam current the implant process can be adjusted to provide that compensation.
In U.S. Pat. No. 4,234,797 there is disclosed apparatus for controlling the treatment of a workpiece by a beam emanating from a source, in which there is translational relative movement in two orthogonal directions between the beam and the workpiece support element, and control of velocity in one (control) direction occurs in response to a detector, mounted behind the support, which periodically samples the beam through a moving slot in the support element. This slot extends over the range of movement in the control direction. An ion implanter is shown in which the support element is a constantly spinning disk the axis of which is translated in the control direction. Another ion implanter is shown in which the support element is a moving belt. A simple control circuit, useful for both embodiments, achieves a uniform ion dosage upon semiconductor substrates at a high production rate despite variations in beam intensity. The detector is not affected by a shower of electrons upon the support that neutralizes charge on the workpieces.
In each of U.S. Pat. Nos. 4,539,217, 4,587,433 and 5,760,409, both method and apparatus are disclosed for measuring and compensating for neutral ions in an ion beam in the dose control system of an ion implanter. The gas pressure in the implantation volume is measured, and the pressure signal is used to calculate an effective or corrected beam current value in accordance with a predetermined relationship between the gas pressure, the apparent or measured beam current and a term which is commonly referred to as the Pressure Compensation factor K. The resulting effective beam current value is then supplied to the dose control system.
In U.S. Pat. No. 4,539,217 and U.S. Pat. No. 4,587,433, a dynamic mode of operation is described in which values for K are determined during an ion implantation process, by comparing simultaneous measurements of measured ionic current and pressure, with corresponding simultaneous measurements taken previously during the process. In these patents, K is taken to be defined by the relationship EQU I.sub.o =I.sub.m (1+KP),
where I.sub.o is the corrected beam current I.sub.m is the measured beam current
and P is the residual pressure.
This assumed linear relationship between corrected beam current and residual gas pressure provides limited accuracy, especially at higher beam energies.
In U.S. Pat. No. 5,760,409, K is defined by the relationship EQU I.sub.m =I.sub.o [1+(.gamma.-1)(1-e.sup.-KP)],
which reduces to I.sub.m =I.sub.o exp(-KP) if .gamma.=0.
The additional parameter .gamma. can be interpreted as the ratio of final steady charge state to the initial injected charge state. Both K and .gamma. are determined empirically prior to performing production implant runs, and the values stored for each particular process recipe to be optimised.
The need to perform repeated test implants in order to assemble values for K and .gamma. for each implant recipe is time consuming and laborious. Also an unexpected change in a parameter during an implant process could result in the computations, using empirically determined K and .gamma. values to calcultate a corrected beam current, becoming inaccurate, resulting in dosimetry errors.